What are the benefits and limitations of the relational database model for business applications?

In theory, relational databases (RDBMS) probably have no "hard" limits. In practice, existing products tend not to perform very well at certain kinds of processing. RDBMS are optimized to perform many different kinds of queries on the same data fairly well. This keeps them flexible, able to handle unanticipated uses.

But some applications require very specific kinds of queries and need these specific queries to be as fast as possible (or at least have well-known response characteristics). Sometimes niche or custom databases perform sufficiently better than existing RDBMS that it warrants such solutions.

Some also suggest that RDBMS don't fit object-oriented applications very well. This is a contentious topic with no easy answer. RDBMS and object-centric designs tend to use conflicting philosophies such that mixing and coordinating them can be difficult and expensive. Some commercial products claim to bridge the gap.